Distribution of UHMWPE wear particles in periprosthetic tissues of total hip replacements

INTRODUCTION: Information on the distribution of UHMWPE wear particles around joint replacements is essential for the understanding of aseptic loosening of implants. MATERIAL AND METHODS: Samples of soft tissue from around total hip arthroplasty (THA), which had a typical appearance of polyethylene granuloma, were collected from the places corresponding to the radiological zones of Gruen and DeLee. The samples were taken from 45 patients (group N1). Since the samples varied in macroscopic appearance and/or the technique of processing, a sub-group of 15 more uniform samples (group N2) were collected by the first author in order to ensure the maxi- mum reliability. The samples were examined for the extent of tissue damage and the amount of particles in each zone. The comprehensive orthopaedic evaluation of tissue damage was carried out as follows: the first author studied the joint replacement revision protocol and pre-operative radiograph in each patient. He evaluated the degree of damage in each zone and wrote down the results in the form of an ordered series according to increasing damage (relational statement). This ordered series was called orthopaedist's statement (OS). The amount of particles in each zone around THA was assessed by the IRc method developed in the laboratory of the second author. IRc method yielded numbers that were proportional to the volume of biologically active particles with size 0.1-10 microm in given zones. The set of numbers from IRc method for each individual case of THA was ordered in a series (relational statement). This ordered series was called a measurement statement (MS) and could be directly compared with orthopaedist's statement (OS). The OS's and MS's for each patient were statistically evaluated, independently for each group (N1=45, N2=15), and two hypotheses were tested: a) distribution of particles in various zones around THA is uniform, b) there is an agreement between OS and MS. RESULTS: The number of UHMWPE wear particles in different periprosthetic zones within each patient was significantly different in both groups (N1 and N2) at the significance level of P=0.001. The OS's and MS's were identical at the significance level P=0.05. Moreover, maximum amount of particles was found in zones III and 7 in 40 patients out of 45. DISCUSSION: Polyethylene wear debris is considered one of the major causes of THR failures. However, neither national nor international literature has provided a study that would show a statistically significant relationship between the numbers of wear particles in individual zones and the extent of tissue damage in these zones. This is most probably due to the fact that four independent methods had to be developed in order to compare OS's and MS's: 1) accurate technique of tissue sampling during THA revisions, 2) accurate method of isolating wear particles from the tissues collected, 3) efficient, reliable and reproducible quantification of UHMWPE particles and 4) correct method of statistical analysis for comparison of the orthopaedist's statements and the measurement statements for all patients involved. CONCLUSION: The distribution of wear particles in the periprosthetic tissues of THA is not uniform, i.e. the amount of wear particles of each patient varies from zone to zone. The extent of tissue damage in each zone, as assessed by the orthopaedic surgeon (OS), was in a good agreement with the volume of 0.1-10 microm particles obtained by IRc measurement (MS) of the zone. We have provided conclusive evidence that the highest number of particles is accumulated in zone III and zone 7. As the number of particles the zones varies and the greatest damage is repeatedly found in areas with the highest particle numbers, it can be concluded with certainly that wear particles are one of the main causes of aseptic joint implant failure.